Long before meeting her, Ryan had a depressive episode which led him to commit a series of bizarre bank robberies in Toronto, Canada dressed as an elderly man.
Because the crimes were so out of character, the judge only sentenced him to three years and nine months in prison, which came down to just 15 months thanks to his good behaviour inside, The Toronto Star reports.
Ryan met Kristen while he was still on parole and the pair quickly fell in love and began planning to wed.
However, Ryan wasn't totally honest with his fiance - he told her he had graduated from college when he had actually dropped out months earlier.
He was then fired from his job when they found out about his history, however, he told Kirsten he was working from home.
Ryan kept up the charade until his mum, Susan, begged him to tell Kristen the truth just a month before their wedding.
Susan told her son she would cut him off financially if he didn't come clean, The Sun reports.
Worried that Kristen would call off the wedding, Ryan decided to carry out an elaborate plan to kill his mother and brothers so the truth about his lies wouldn't get out.
He hid a crossbow in his mum's garage, leaving an iPad and iPhone at his house to try to create a digital footprint which would place him at a different location.
Following an explosive argument, Ryan killed his mum and waited for his brothers to arrive. Ryan fatally shot his two brothers before the third arrived and managed to escape and call for help.
When emergency services arrived, Brett Ryan confessed to all three murders, News.com.au reports.
Last Friday, the 36-year-old was sentenced to three life sentences without the possibility of parole.